Percipio - ITPro TV - TCP/IP & Networking

Linux - Network operating system.

How are IP address assigned?

IP's are similar to phone numbers. Computer has IP assigned to them and it has to come from somewhere.

ifconfig

Gives IP address but it is older command.

ip addr

Above command shows IP address. DHCP server will hand out the address. When the computer is started, it asks for IP address and DHCP server responds to it. DHCP do DORA (discover, offer, request, acknowledgement). So when our laptop is started it do discovery and ask could somebody please give me an IP? This message is sent out and DHCP server will hear it and respond back with offer. Our laptop will request to keep the IP that is offered by DHCP server. Finally DHCP server will assign the IP to our server. These are usually automatic and we can even do it manually. IP address come from 32 bit address pool. In our IP /8, /16 or /24 will tell us which class we belong to. We are not following this anymore.

3 types of network

Class A - 255.0.0.0 - /8 - can hold 16 m hosts (255 is network and 0 is host, so if my ip is class A first digit is network and remaining 3 is host, in class b first and second digit is network and 3 and 4 digit is host. It depends on which class I am in)

Class B - 255.255.0.0 -/16 - can hold 65,534 hosts

Class C - 255.255.255.0 - /24 - can hold 254 hosts (common in home network)

In IP address part of it represent network and part of it represent the host. It is like the street address. Lets take an address 71, Ashraya Layout. So there can many house in Ashraya Layout it can be compared with network. However, 71 is specific to our home and it is equivalent to host.

How do we choose which addresses to use?

Private IP can be pick amongst your heart's content. Public IP has to be globally unique. ARIN is the organization register number and they make sure there is no duplicate. On LAN we will have fake ID and NAT Device will translate to public IP. Below ranges are set aside and we can pick any IP

RFC 1918

198.168.x.x/24 - We can use any IP inside it

172.16.x.x -> 172.31.x.x/16

10.x.x.x/8

What is IPV6?

IPV4 -> 32 bit address. 4.3 billion address (earth population was also 4.3 bil and not many had computers)

Now it is around 7billion and many have computers.

IPV6 -> 128 bit address (1 decillion address). With IPV6 we can give 4.3 billion address to each individual

Where does TCP come in?

IP is responsible for addressing, where is your traffic coming from and where is the traffic going to. TCP allows us to establish session.

nslookup towel.blinkenlights.nl (will show the IP)

telnet towel.bilinkenlights.nl

ss -an (we can use to see above session, ss stands for socket statistics, when we open session then it is call socket)

a-> all session

n-> don’t do named lookup

Telnet will have port 23

What port number we should be familiar with?

Some protocol uses both TCP and UDP. FTP uses two port one for commands and another for transferring data. DNS already have re-transmission built-in so we don’t need TCP for that.

20	FTP	TCP
21	FTP	TCP
22	SSH	TCP
25	SMTP	TCP
53	DNS	TCP/UDP
80	HTTP	TCP
110	POP3	TCP
119	NNTP	TCP
123	NTP	UDP
139	NetBios	TCP/UDP
143	IMAP	TCP
161	SNMP	UDP
443	HTTPS	TCP
465	SMTP-SSL	TCP
993	IMAPS	TCP
995	POP3S	TCP

How networking configured in Linux Server: - Video 2

How to view our current network configuration?

15 years ago it is pain, now it is mostly automated.

Different ways are there to see current configuration.

ifconfig  (It is older tools and it wont come by default in newer version)

We see all network adapters and their addresses. If the network adaptor is ON and we have DHCP serer they we see the IP.

route (this command show routint table but it is old)

IP commands are newer and we should start learning this compared to ifconfig and route

ip addr (it shows address information)

ip route (to see routing information)

Is there anything that we should do before we start changing things?

For simple problem like, we set DHCP and not getting DHCP address we just need to restart. DHCP service is not running. Or our dhclient is not running. We can do by running below command.

sudo dhclient

If it is already running we can do retry

sudo dhclient -r

To restart entire network stack

For systemd based system

sudo systemctl restart network

For Sysvinit based system

service network restart

Where are the network interface configurations stored?

In Redhat

Network configuration are stored in /etc/sysconfig/network-scripts We will have network scripts. Each interface have own script. They will have network settings.

Meaning of letter in the name:

en -ethernet

wl -wireless

ww - cellular (wireless wan)

p - pci

o - onboard

s - hot plug support

We can switch back to static IP by using following config

BOOTPROTO=none

IPADDR0=<IP Address you wish to have>

PREFIX0=16 (subnet)

GATEWAY0=10.0.0.1

To take the settings to take effect, restart network stack (network restart) or full OS.

In Ubuntu

/etc/network -> Used to store here

/etc/netplan -> can be here too

Check the interfaces file inside both folders. It should show the interface that we get using "ip addr" command in that file. If it only has 'iface lo inet loopback' then that is not the right one. Under netplan, you will only find yaml file. Look for 99_config.yaml. You should see the interface that we get using "ip addr" here. When using netplan we don't need to restart. It applies the settings directly without restart.

Command to use when netplan config is changed:

sudo netplan apply

What about global settings like DNS servers?

In Centos:

/etc/sysconfig, there is a file called network.

/etc/resolv.conf has DNS. By default it will pull from DHCP server. But we can also change it.

/etc/hosts will override resolv.conf. It has IP address at left and domain name at right.

/etc/hostname is not global settings. It is where our computer name come from. If we edit this file and reboot the settings will be lost. To make permenant changes use hostnamectl command.

sudo hostnamectl set-hostname Centos

It doesn't obey capitilization

Are there any easier ways to configure network settings?

It is easier using GUI when we have desktop version. If we don't have GUI many distro supports network manager. To find out if we have network manager, launch nmcli. It gives command lines interface and takes care everything in backend. It know where the config are present.

nmcli device status

After running above command it should show us some output like DEVICE, TYPE, STATE, CONNECTION with green font. If it doesn’t show any output then we are doing manual configuration. The state will show Connected for which nmcli is managing for others it shows unmanaged.

In Ubuntu

nmcli device status

It shows everything is unmanaged because of netplan migration. First understand whether the GUI is netplan or network manager. Based on that use the same method. If it is netplan use netplan. If it is network manager then use it. You can find whether it is managed using network manager or not using nmcli device status command.

Adaptername - etho

nmcli device show <adaptername>

To change any settings:

nmcli connection edit <adaptername>

The above command will directly take to adapter. You can give below command inside the adapter

set connection.autoconnect yes

set ipv4.method manual

After running above command use save command to save. But it will lost after reboot. If you want even after reboot, use save persistent to save it to hard drive.

Do, sudo nmcli connection reload

The above reload the connection. We notice there are lot of commands but it is best practice as we don't need to chase for files.